This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: On Jan. 3, 1117, a strong earthquake shook northern Italy. A manuscript of the 12th century, kept today in the Vatican Library, recorded what came next: "The lakes also rose so much that very few of those living in the villages nearby escaped this danger." Lakes are rarely thought of as places for tsunamis.
Yet the delta that a river builds in a lake can collapse, and the sediment sliding into deep water pushes the water above it into waves. In our article published in Nature Communications, we show that at Lake Iseo, people increased the risk of such collapses for centuries without knowing it, and that the danger has returned. The study was led by Muhammad Naveed Zafar, whose doctoral thesis on lake tsunamis I co-supervised with the geologist Pierre Sabatier at the University Savoie Mont Blanc, in France.
With William Rapuc, Raphaël Paris, Andrea Piccin, Pascal Vuillemin and Flavio Anselmetti, we combined geology, history and computer simulations. My own part was the mathematics of the waves. Lake Iseo lies at the foot of Val Camonica, one of the largest valleys of the southern Alps.
Its river, the Oglio, has built a large delta at the lake's northern end. Since Roman times, forest clearing, farming and grazing have exposed the slopes of the valley to erosion, tripling the sediment that reaches the lake. The delta grew quickly, and its underwater front became steep and overloaded.
It had collapsed once already, in the sixth century, and six centuries of this erosion loaded it again. Two earthquakes, one in 1117 and one on Christmas Day in 1222, shook the overloaded front loose. Sediment cores and seismic profiles, which image the layers under the lake floor with sound, show both collapses as the thickest layers in the deep basin.
The first moved about 62 million cubic meters (81 million cubic yards) of sediment, enough to fill a cube nearly 400 meters (1,310 feet) on each side. The second, a century later, moved about 10 million cubic meters (13 million cubic yards): the river, still fed by erosion, had rebuilt the front in between. To turn these layers into waves, Naveed took today's delta as the shape before each collapse and carved into it a scar holding the volume found in the deep basin.
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